Various optical fiber cables are known. For example, U.S. Pat. No. 6,572,081 describes an optical fiber cable and a method for installing guide tubes through which optic communication cables are routed. Such cables are generally installed underground such that the main tube is accessible via its ends.
The optical fiber cable according to the present invention may also be defined as a tube system for optical fiber cables, wherein a tube, also known as “main duct,” of such a tube system corresponds to the main tube of the present application. Furthermore, the guide tubes according to the present invention are also referred to as “micro ducts.”
After installation of the main tube, the number of guide tubes and the optical waveguides that may be present therein may be adapted to the current need. It is also possible to add guide tubes to the hollow space of the main tube and optical waveguides to the guide tubes.
Guide tubes containing optical waveguides may also be installed prior to the installation of the main tube. Usually, however, the main tube is installed first and subsequently the guide tubes are installed via the blow, push, float, or draw method. Optical waveguides can be similarly installed in the guide tubes via the blow, push, float or draw method.
A drawback of the optical fiber cables that are presently commercially available is that the main tube (and possibly the guide tubes), after installation, may be insufficiently water resistant (e.g., leak-tight). Consequently, there is a possibility that water will enter the main tube (or the guide tubes).
One cause of such water ingress (i.e., in-leakage) is that the main tube and/or the guide tubes are damaged before, during, or after installation. Another cause of water ingress is that connections between various main tubes and/or guide tubes in a cable network are not watertight. In addition, there is a risk of water remaining behind in the main tube when the so-called float method is used for installing guide tubes or optical waveguides. This may lead to adverse effects.
One problem with the presence of water in the hollow space of the main tube is that the water may freeze. The expansion of water upon freezing exerts pressure both on the wall of the main tube (i.e., inside-out pressure) and pressure on the guide tubes (i.e., outside-in pressure). Pressure from ice will also be exerted on the optical waveguides present in the guide tubes. Consequently, the signal being conveyed through the optical waveguides will be disturbed, which in turn will cause the communication network (of which the cable forms a part) to malfunction.
Moreover, if the damage from freezing water is permanent, it may be necessary to repair the cable and/or replace the optical waveguides. This, of course, involves considerable expense.
German Publication No. 2842604 discloses a method for dimensioning the wall of a tube in which optical waveguides are present such that the adverse effects of the freezing of water are eliminated.
It is known in the art to employ water-absorbing or water-blocking materials (e.g., tape, yarn, gel, powder, or combinations thereof) to reduce or prevent the ingress of water into optical fiber cables.
For example, U.S. Pat. No. 4,815,813 discloses water-blocking strips or yarns that may be wrapped around or be provided parallel to a central tube.
U.S. Pat. No. 6,321,012 (and counterpart European Patent Application No. 1,081,519 A1) discloses the use of a water-swellable binding yarn or tape wrapped around a bundle of optical fibers or around tubes in which optical fibers are present.
U.S. Pat. No. 6,633,709 discloses a cable wherein water-blocking yarns are provided around stacked fiber optic ribbons with or without the use of a water-absorbing powder.
European Patent Application No. 1,302,797 A1 discloses an optical fiber cable configuration having a tubular outer jacket that houses a plurality of buffer tubes containing optical fiber ribbons. A central strength member is positioned concentrically with respect to the outer jacket and includes a strength tube, which can change in shape upon application of force on the optical fiber cable configuration.
The foregoing patents and publications are hereby incorporated by reference in their entirety.
The use of one or more water absorption materials involves additional expense and, in addition, makes it more difficult to install guide tubes containing optical waveguides in the main tube via the blow, push, float, or draw method. When a water-absorbing gel is introduced into the hollow space, for example, the frictional resistance that is subsequently generated between a guide tube to be installed and the aforementioned gel will be so high that only short installation distances can be bridged. This means that the length of the main tube is limited. The consequence is that it is necessary to couple successive main tubes more frequently, which is undesirable, as it involves an increased risk of leakage.
Therefore, there is a need for an optical fiber cable whose designs limits the adverse effects of freezing water within its main tube.